1. Field of the Invention
This invention relates generally to a cathode ray tube for reproducing a picture on a phosphor screen, and more particularly to an improved cathode ray tube in which a landing spot of an electron beam on a screen is compensated for in distortion of its shape caused by the deflection of the beam.
2. Description of the Prior Art
It has been known that in a cathode ray tube, especially, of the type of wide beam deflection angle, an electron beam is given a deformed cross section by a deflection magnetic field through which the electron beam passes toward a phosphor screen and thereby the beam landing spot on the screen at its some areas is so distorted as to include a portion having an increased current density and as a result, the phosphor at such a portion of the beam landing spot having increased current density is brought in the luminosity-saturated condition to introduce an undesirable picture without high fidelity. By this distortion due to the magnetic deflection field, the beam landing spot at the peripheral portion of the screen is formed in a long sideways oval shape, while the beam spot at the central portion of the screen is formed in a desired circle shape.
For compensating for such a distortion in shape of the beam landing spot, there have been proposed some ways. For example, a magnetic four-pole device is provide on the neck portion of the cathode ray tube for forming the cross section of the beam into a proper shape with a magnetic field produced by a current flowing through the device in synchronism with the deflection current or an electrostatic four-pole device is provided at a focus electrode of an electron gun assembly in the tube for forming the cross section of the beam into the proper shape at the peripheral portions of the screen with an electric field produced at the device. However, these conventional ways require relatively much power consumption and accordingly there is a difficulty for them to be actually employed in commercial models.